Within the past few years there has been added to the electron impact ionization (EI) mode of ionizing substances in the gaseous state for mass spectrometric analysis, another mode of ionizing, called chemical ionization (CI). Whereas in electron impact ionization the substance under investigation is bombarded with electrons, in chemical ionization the substance is reacted with a known pre-selected set of reactant ions. In practice, chemical ionization is achieved by forming primary ions in the ion source by electron impact ionization of a reactant gas at pressures greatly in excess of those used in electron impact ionization of substances (e.g., about 1 Torr for CI vs. about 10.sup..sup.-4 Torr or less for EI). The primary ions thereby formed undergo ion-molecule reactions with neutral reactant gas molecules so as to produce reactant ions. Finally, chemical ionization of the substance under investigation occurs in the same ion source by ion-molecule reactions of the substance with the reactant ions. Of course, all of these three steps occur substantially simultaneously and continually in the CI source. In general, chemical ionization mass spectra have fewer ions and more intense high mass ions than electron impact mass spectra and the two ionization techniques are highly desirable complementary analytical tools. A comparison and further analysis of these techniques may be found in applicant's article "Chemical Ionization Mass Spectrometry," appearing as Chapter 31 of "Biochemical Application of Mass Spectrometry" edited by G. R. Waller, Wiley-Interscience Publishers, New York, N.Y., 1971.
Because of the complementary nature of the two ionization techniques, attempts have been made to provide a single source capable of both chemical and electron impact ionization at consecutive times, with the reactant gas pressure being the determinant of whether the source is operating in a CI or an EI mode. However, because the CI pressures are considerably above the typical pressure in the analyzer region of the mass spectrometer (usually less than 10.sup..sup.-5 Torr), a gas-tight ion source region must be provided, requiring that the electron entrance and ion exit apertures be very small. This reduces significantly (by roughly an order of magnitude) the sensitivity of the source in the electron impact mode of operation, simply due to the decrease in the number of electrons entering the source through the narrow electron entrance aperture and the decrease in the number of ions leaving the source through the narrow ion exit aperture.
Moreover, when the ion source is fed from the outlet of a gas chromatographic column, an analytical technique gaining much importance wherein, in effect, the mass spectrometer becomes the detector for gas chromatography, additional problems of the combined CI-EI source surface. For chemical ionization, the technique is simple since the effluent from the gas chromatographic column may be fed directly to the ion source of the mass spectrometer, with the carrier gas of the effluent serving as reactant gas in the ion source. However, if the effluent is to be subjected instead to electron-impact ionization, a splitter or a separator must be provided at the gas chromatography--mass spectrometer interface, thereby complicating an otherwise simple interface. An alternative is to reduce the effluent flow rate drastically to keep the pressure in the ion source at the level (10.sup..sup.-4 Torr and Lower) required for electron impact ionization. Such flow rate changes make difficult correlation between the CI and EI spectra of the effluent. Moreover, because of the large disparity in pressures required for the EI and CI modes, switchover in the middle of a gas chromatographic peak (typically of 15 seconds duration at half height) is not possible. Thus separate time-separated runs are needed on duplicate sample injections and hence, despite all efforts at identity and ignoring the considerable time loss in such duplicate injections, this leads, minimally, to a great deal of effort in correlation of the EI and CI spectra.